The broad goal of this research is to learn how a single epithelial cell sheet becomes patterned during embryonic development into discrete domains of different cell types. The epidermis of the nematode Caenorhabditis elegans, will be studied as a simple model epithelium. This epithelium becomes subdivided into three major cell subtypes during embryogenesis. Particular emphasis will be placed on the mechanisms by which one of these subtypes, the seam cells, are specified. Because the mutations studied here lead to severe defects in embryonic development, these studies should serve as models for human birth defects. In addition, by examining genes that cause proliferating embryonic cells to select particular paths of differentiation, this work should contribute to our understanding of mechanisms by which normal controls of growth and differentiation are abrogated in malignantly transformed cells, resulting in cancer. The zygotically expressed gene, zen-3, is essential for normal epidermal patterning and differentiation of seam cells during embryogenesis. Patterning and seam differentiation defects in six zen-3 mutants will be studied by immunofluorescence with epidermis-specific antibodies to characterize the range of defects resulting from changes in the activity of zen-3. The development of individual epidermal cells in a representative zen-3 mutant will be followed by cell lineage analysis using a video system that records the position of all cells throughout embryogenesis. To assess the time of action of zen-3 function, the temperature-sensitive (ts) period of a ts zen-3 mutant will be determined. A yeast artificial chromosome that carries zen-3 (+) function will be identified and fused to a duplication carrying a cell-autonomous marker. This fused duplication will be used to define the cells in which zen-3 is required by performing genetic mosaic analysis. To initiate studies of other genes involved in seam specification and epidermal patterning, an existing collection of 55 mutants defective in epidermal development will be screened by immunofluorescence and time-lapse. This screen will be used to narrow in on one or a few genes for future detailed investigations. Finally, laser ablations of early blastomeres will be performed on wild-type embryos to identify cells required for seam cell differentiation. These experiments will ultimately lead to the elucidation of epidermal patterning at molecular resolution.